Implications of Soil Biodiversity for Sustainable Organic Matter Management - Sustainable Management of Soil Organic Matter - page 380

Fig. 5.5.1. Soil microbial biomass N at increasing distance from the root mat. Bars indicate SE

(n= 4).

In the fertile soil, SMB 14 C in the rhizosphere increased from 91 pg g −1

soil in the bulk soil to > 250 pg g −1 soil at the rhizoplane; in the

nutrient-depleted soil, no rhizosphere effect was observed (Fig. 5.5.2).

Discussion

The measured extension of the rhizosphere in this experiment (1-3 mm) is
in accordance with findings of previous studies, whereas the increase in
SMB at the rhizoplane is in the low end of the measured range (Helal and
Sauerbeck, 1986; Youssef et al ., 1989; Yeates and Darrah, 1991; Badalucco
et al ., 1996). However, direct comparison between the studies is not
possible due to variation in experimental set up and choice of parameters.
Data from harvest at day 8 (data not shown) indicate that the SMB was no
longer increasing rapidly; however, the incubation time was shorter than in
most of the compared studies. The less pronounced rhizosphere effect in
the nutrient-depleted soil was most likely caused by lack of available nutrients
for microbial growth.
Studies of the effect of live plant roots on decomposition of SOM
show contradicting results. Several studies found that

14 CO 2 evolution